1. Field of the Invention
This invention relates to an ultrasonic measuring method and an ultrasonic measuring apparatus for examining the inside of an object by the ultrasonic pulse echo method.
2. Description of the Related Art
The ultrasonic pulse echo method is well known as a method of detecting any cracks or peelings inside an object without breaking it.
The principle of the ultrasonic pulse echo method will be illustrated below along with the process for obtaining an ultrasonic image through this method.
A sound wave is partly reflected by an acoustically discontinuous surface (boundary surface). The reflectance R can be expressed as: EQU R=.vertline.(Z.sub.2 -Z.sub.1)/(Z.sub.2 +Z.sub.1).vertline. (1)
wherein Z.sub.1 expresses the material impedance on the incidence side, and Z.sub.2 that on the transmission side. Further, it is known that in the case of a reflection on a boundary surface where Z.sub.2 &lt;Z.sub.1, a phase inversion occurs between the incidence and the reflected waves. In the ultrasonic pulse echo method, an ultrasonic pulse is transmitted to the object to be examined to generate a reflection signal. By passing the reflection signal through a time gate of a suitable time constant, the reflection signal which comes from the inside of the object can be extracted. The inner condition of the object can be examined by measuring the level of the reflection signal thus extracted.
Considered here will be how any peeling between plastic and metal surfaces can be detected by this method.
A transmission pulse sent from an ultrasonic transducer is partly transmitted through the plastic surface to become an incidence wave on the boundary surface between the plastic and metal surfaces, the phase of the transmission pulse being maintained. The reflectance on the boundary surface between the plastic and metal surfaces is ca. 50% as given by the above equation (1), no phase inversion occurring since Z.sub.1 &lt;Z.sub.2. When an air layer is generated as a result of peeling, the ultrasonic pulse is reflected by the boundary surface between the air and the plastic surface. Since the reflectance is about 100%, and Z.sub.1 &gt;Z.sub.2, a phase inversion occurs. In an actual measurement, it is difficult to measure the absolute reflectance because of the damping of the sound wave due to absorption, dispersion and the like. Accordingly, peelings are detected by comparing the level of the reflection signal from them with that of the signal from the close-contact area.
However, if the covering of the entire measurement area has been peeled off, it is difficult, in the above-described conventional method, to judge whether the area is in the peeled or the close-contact condition. In the case of a fine peeling in which peelings partially exist within the diameter of the ultrasonic beam, the signal with no phase inversion from the close-contact area and that with inverted phase from the peeling area may combine with each other to form a synthetic wave, canceling each other to lower the signal level. As a result, the signal level of the synthetic wave is lower than that of the wave from the close-contact area, thereby leading to confusion in discriminating between close-contact and peeling areas.
The above problem is eliminated in a conventional measuring method by using a single-shot pulse with an asymmetrical waveform as the transmission pulse. According to this conventional method, the positive and the negative peaks of the reception signal are caught so as to obtain the difference in absolute value between them as the reflection signal level, which includes the phase inversion. The problem with an apparatus using this method is that the difference between the peak values is rather small when the waveform of the transmission signal is not sufficiently asymmetrical, so that it is difficult to detect. Furthermore, when the asymmetry is lessened by any irregularity in the waveform due to the condition of the boundary surface, the difference between the positive and negative peak values becomes smaller in spite of the high magnitude of the signal itself, so that the signal level, which is represented by the difference, is lowered, which inevitably makes it difficult to discriminate between the close-contact and the peeling areas.